This invention relates to improvements in kaolin clay pigments chemically bulked by addition of a cationic polymer and, particularly, to improvements wherein the cationically bulked kaolin clay is mixed with a calcium carbonate pigment. More specifically, the invention relates to means for avoiding an undesirable increase in viscosity of high solids dispersed aqueous slurry of cationically bulked kaolin clay when mixed with a calcium carbonate pigment.
It has been well known for many years that the utility of kaolin clay as a pigment to coat or fill paper can be improved significantly by bulking the kaolin with a cationic polymer. See, for example, commonly assigned U.S. Pat. No. 4,738,726, Pratt et al, U.S. Pat. No. 4,772,332, Nemeh et al and U.S. Pat. No. 4,859,246, Sennett. These bulked kaolins are sometimes referred to as xe2x80x9cChemically Engineered Kaolinsxe2x80x9d (CEK). One of the difficulties encountered in commercializing the cationically bulked kaolin clay involves providing concentrated (high solids) aqueous slurries of the bulked kaolin that possess the rheology especially desired by the paper industry. Considerable experimentation has been carried out in the past to provide suitable dispersant systems to achieve the desired rheology. See, for example, commonly assigned U.S. Pat. No. 4,772,332, Nemeh et al, U.S. Pat. No. 4,859,246, Sennett, and PCT, WO99/15596, Willis et al.
For many years, a polydiallyl polymer (referred to as polydadmac) was used commercially as the cationic polymer. More recently, it was found that epichlorohydrin amine condensates were superior in some respects. The polyamine cationic polymers were considered capable of providing higher solids fluid slurries.
Unexpectedly, we discovered that high solids dispersed slurries of chemically bulked kaolin produced with epichlorohydrin amine condensates tended to increase significantly in viscosity when mixed with calcium carbonate pigments. This did not occur when polydadmac was used.
Past efforts to improve the viscosity (fluidity) of slurries of cationically bulked kaolin pigment, have centered on the selection of dispersants. Specifically, anionic sulfonate dispersants, especially when used in combination with polyacrylate dispersants, were found to be advantageous. See the aforementioned commonly assigned patents and publications. For example, U.S. Pat. No. 4,859,246 discloses the use of lignosulfonates and naphthalene formaldehyde sulfonates (Lomar(copyright)D) for this purpose. Illustration examples used polydadmac. High shear (using WARING BLENDOR(copyright) mixer) was employed in lab tests; lower shear (Cowles mixer) were used in pilot plant tests. Levels of sulfonates up to 0.1% by weight of dry clay were used in examples. U.S. Pat. No. 4,772,332 (supra) teaches the desirability of using combinations of ligno and naphthalene sulfonates but notes the adverse effect of lignosulfonate on pigment brightness. See col. 6, lines 51 et seq. U.S. Pat. No. 4,772,332 teaches the use of 0.05% to 0.15% of sulfonate. WO 99/15566 discloses the use with an epichlorohydrin amine bulking agent of 8#/ton (0.4% of dry kaolin) of a dispersant mixture of sodium polyarylate, lignosulfonate and naphthalene sulfonate in unidentified proportions. No carbonate pigment was present.
It has now been discovered that high solids slurries of kaolin clay bulked with an epichlorohydrin amine condensate can be made more compatible with calcium carbonate pigments provided the dispersant used to disperse the chemically bulked kaolin has a relatively high content of a naphthalene sulfonate complex dispersant, (0.20 to 0.30% based on the dry weight of the bulked kaolin) and lignosulfonate dispersants are excluded. From 0.10 to 0.16% by weight of an acrylate salt co-dispersant must also be present. The amount of calcium carbonate pigment is from 1% to 40% based on the dry weight of the bulked kaolin.